Articles manufactured from poly(alkylene terephthalates) have many valuable characteristics, including strength, toughness, solvent resistance, high gloss, and the like. These articles may be fabricated by a number of well-known techniques, including injection molding, roto molding, blow molding, extrusion, and the like, depending on the shape of the desired product.
Certain of these techniques, in particular, blow molding and extrusion, require that the molten poly(alkylene terephthalates) have a suitably high melt viscosity, e.g., in excess of 10,000 poises, to prevent collapse or blow-outs in the soft preformed state. It has been found that poly(alkylene terephthalates) of such high melt viscosity are obtained only with great difficulty in the conventional bulk melt polymerization processes generally used to prepare the polyester.
It has been found that branching the poly(alkylene terephthalates) causes a desirable increase in melt viscosity and melt elasticity. Such branched materials have been made by adding a branching component to the ester-forming ingredients or to the low molecular weight prepolymers normally produced in making linear polyesters.
A method has now been found which permits the conversion of standard grade linear polyesters to branched copolyesters and this eliminates the need to make special branched polyester grades in commercial polymerization equipment.
By way of illustration, branched poly(1,4-butylene terephthalates) or mixed poly(ethylene terephthalate) and poly(1,4-butylene terephthalates) can be made according to this invention from a linear poly(1,4-butylene terephthalate) or mixture thereof with poly(ethylene terephthalate) and one or more branching agents containing three or more ester-forming functional groups by intimately blending the linear polyester or mixed polyesters and the branching agent, by extrusion, milling or other suitable means; followed by solid state polymerization of the mixture at a temperature below its melting point in a vacuum or in a stream of inert gas.
For instance, poly(1,4-butylene terephthalate) or mixed poly(ethylene terephthalate) and poly(butylene terephthalate) or intrinsic viscosity lower than 1.05 dl./g. may be blended with 0.15% by weight of pentaerythritol and the extrudate granulated. The granules are placed in a suitable solid state polymerization unit and heated to 190.degree. to 210.degree. C. in a stream of dry, inert gas until the intrinsic viscosity has increased to at least 1.1 dl./g. or higher. It is found that the pentaerythritol has reacted with the linear polyester to yield a branched polymer, characterized by its high melt viscosity and the rubbery nature of its melt. These characteristics make such polyesters particularly suited for blow molding, extrusion and plastic foam applications.